# Applications of Newton's laws

## Homework Statement

You've attached a bungee cord to a wagon and are using it to pull your little sister while you take her for a jaunt. The bungee's unstretched length is 1.3m and you happen to know that your little sister weighs 220N and the wagon weighs 75N. Crossing a street, you accelerate from rest to your normal walking speed of 1.5m/s in 2.0s, and you notice that while you are accelerating, the bungees length increases to about 2.0 cm. what is the force constant of the bungee cord, assuming it obeys hooke's law?

f=ma
fs=kx

## The Attempt at a Solution

Ok, so I am quite stumped with this question, i know you should find the total force of the little sister and wagon , 220+75, but i do not know if i should keep it as a force or as a total mass in kg. And the difference in spring length 2.0-1.3=.7m . I am just unaware if i bring kinematic forumlas into this and i am quite unaware of what to find because i am getting the wrong answer when looking for "k"

Any guidance is appreciated. Thanks

## Answers and Replies

mfb
Mentor
The problem has a serious flaw, but we can ignore that for now.

I guess the 2.0 cm mean 2.0 m. The weight is a force downwards - as weight it won't help you much as your bungee cord is horizontal. Finding the mass is a good start.
What is the force needed to stretch the bungee cord? What does that force act on?

yeah its 2.0 m . long day lol . ok , so it would be total mass over gravity, giving you; 295/9.80= 30.102kg ? acceleration is 1.5/2 = 0.75 m/s^2. I dont get what you are asking in the second part though.

cant believe i had that brain fart. thanks !

mfb
Mentor
Let me rephrase that: Why does the wagon accelerate, and which force (in N) comes from where?

Edit: Okay, solved?

from the pull? and i multiplied the mass and acceleration and divided by the .7m difference in the spring?

mfb
Mentor
Looks fine.